Artificial implants, including hip joints, shoulder joints, and knee joints, are widely used in orthopedic surgery. Artificial hip and shoulder joints are generally ball and socket joints, designed to match as closely as possible the function of the natural joint. To duplicate a joint's natural action, a total joint replacement implant has three parts: the stem, which fits into the femur or humerus and provides stability; the ball, which replaces the spherical head of the femur or humerus; and the cup, which replaces the worn-out hip or shoulder socket.
There are many types of stems that may be used in joint replacement surgery to secure the artificial ball that will articulate with the artificial socket. Some stems are modular, allowing a greater range of options during the surgery. Each part comes in various sizes in order to accommodate various body sizes and types. In some designs, the stem and ball are one piece; in other designs, they may be provided as separate pieces. In further designs, the stem and ball may feature a modular body, a removable neck, or any combination of these or additional features. Such designs and their combinations will be referred to throughout this document as “modular,” and are intended to allow for additional customization and fit.
Specifically, modular stems may be provided in any number of lengths and widths. Corresponding modular bodies and heads may be provided in various sizes, allowing the surgeon to select the best options for the particular patient. Other stems may be non-modular, and may provide a stem, neck, and ball in a one-piece configuration.
In choosing a proper implant, the surgeon will consider individual patient needs. One consideration may be modularity; another consideration may be stability needed based on proximal bone degeneration that may have already taken place. If there is substantial bone degeneration, the surgeon will likely choose a stem with additional distal fixation features to prevent any more proximal bone erosion. In other words, the surgeon must seek supportive bone more distally, rather than relying on support from the proximal bone. This is particularly an issue in revision surgeries, although it may also be a concern during a primary surgery. Accordingly, there is a challenge to provide stems that are stable and provide efficient loading for distribution and implant longevity, but that also allow the surgeon as may options as possible.
The two current, but different, distal femoral fixation philosophies include relying on (a) a cylindrical coated stem (the coating may be a biological coating or a bead coating, described more fully below) or (b) a tapered grit-blasted stem. Both types of stems have disadvantages.
Some surgeons using traditional, conical, tapered hip stems find it difficult to recreate the femoral neck height because the stem locks into place within the femoral canal too high or too low. The primary factors resulting in stem placement uncertainty are the difficulty in reproducible reamer depth placement and variations in bone quality. Some revision tapered hip stems offer modular necks of various heights to compensate for the difficulty, but offering modular necks as the sole solution may increase the complexity of the surgical procedure.
Cylindrical biologically and/or bead coated revision hip stems have the advantage of a more reproducible fit during the surgery. However, the biological enhancement coating in conjunction with the cylindrical shape of the implant may cause stress-shielding to the proximal bone due to the concentration of loading forces in the mid to distal portion of the stem. (Unlike a tapered stem, a cylindrical stem loads the bone entirely against shear forces within the femoral canal. The inefficient loading distribution likely causes bone loss proximally, making future revision surgeries even more difficult.)
Cylindrical stems are typically bowed stems. Recently, another design that has been explored is a tapered stem with a cylindrical bow at its distal end. When seeking support for the prosthesis more distally, a surgeon may alternatively use a longer revision stem. With long stems, the most distal section of the stem must be modified to accommodate the anterior bow of the femur. This is because, although a longer stem can provide additional stability, the length of the stem could perforate the cortex of the femur during insertion if the stem is not slightly curved to follow the natural curve of the femur. Current options for modifying the stem are bowing, angling, or otherwise shaping the distal section of the stem to follow the natural curve of the femur, or adding a chamfer slot in the stem. Accordingly, although bowed stems can provide increased stability, they can limit stem neck orientation options because the one-piece options do not always provide the modularity that may be desired at times due to the stem being limited to being orientated in the proper direction of the bow. If an independent body and/or neck portion is provided for use with the stem, orientation options may be increased, but as mentioned, this may also increase the complexity of the surgery.
Other stems designs seek to provide additional stability by offering bone in-growth or bone on-growth surfaces. These surfaces can be physically formed by roughening the stem surface, providing metallic beads, grit blasting, or forming a porous surface in the stem surface such that it supports bone growth. Additionally or alternatively, the surface of the implant may be provided with hydroxyapatite or any other biological substance that promotes bone in-growth or on-growth.
Other attempts to provide stems with greater stability have included providing grooves at certain areas on the shaft portion. The grooves are typically intended to vary the stiffness along the stem shaft to provide stiffness and stability in some areas but less stiffness in other areas.
However, there is a need in the art to provide an intramedullary stem that provides strong distal fixation (particularly for revision surgeries when there is not much bone at the proximal portion of the bone). There is also a need for a stem with the desired distal fixation, but that also provides flexible options that are not limited by stem length and the requirement of a bowed portion at the distal end of the stem. There is also a need for a stem that has axial and rotational stability, while also providing a certain amount of flex. There is also a need for a stem that provides increased surface area for bone in-growth and/or on-growth.